Holiday calendar: Season's tiltings

This picture of Earth was taken at 06:00 GMT on Dec. 21 by Eumetsat's Meteosat-9, a meteorological satellite that is stationed in geosynchronous orbit above a point close to Africa's west coast. The picture illustrates how Earth's tilt with respect to the sun creates the darkest night of the year for the Northern Hemisphere, and the longest stretch of daylight for the Southern Hemisphere.

Tonight is the longest night of the year for the Northern Hemisphere, due to the winter solstice. The season officially changes from autumn to winter at 12:30 a.m. ET Thursday ... unless you're south of the equator. In that case, spring is turning to summer.

You probably learned in school why the seasons (and the temperatures) change during the course of the year, but in case you need a refresher on how the 23.5-degree tilt of Earth's axis affects the weather, we have the full story for you. This picture, snapped by Eumetsat's Meteosat-9 weather satellite today, shows the situation graphically.

Meteosat-9 is camped out in a geosynchronous orbit that puts it precisely above an equatorial point on the west coast of Africa. Every day at around 6 a.m. local time, it has a great view of the terminator line between day and night, cutting straight across Earth's disk. The slant of that line changes from day to day, due to the changing orientation of Earth's tilted axis with respect to the sun.

On the day of the December solstice, the slant is at its most extreme angle, leaving the north pole in the dark while exposing the south pole to 24 hours of daylight. That's what you're seeing in the photo above. National borders and crosshairs have been added to help you get oriented properly.

This video, put together by NASA's Goddard Space Flight Center, shows you how the slant changes from one September equinox to the next:

A NASA compilation of Meteosat imagery shows how Earth's terminator line between day and night changes over the course of a year.

If you check out Eumetsat's near-real-time imagery from Meteosat-9 for 1 a.m. ET (06:00 GMT) Thursday, you can see the solstice effect pretty much at its peak. These pictures of the shifting seasons serve as tonight's offering from the Cosmic Log Space Advent Calendar, which has been featuring daily images of Earth from space during the run-up to Christmas. Feel free to click through these previous images in the series, and check back on Thursday for another satellite image that will take the edge off winter.